The present invention relates to a process for synthesizing a metal oxide having a perovskite or perovskite-like crystal structure by high energy milling. More particularly, a mixture of starting powders are subjected to a high energy milling sufficient to induce chemical reaction of the components and thereby directly mechanosynthesize a metal oxide in the form of a perovskite or perovskite-like nanocrystalline structure as determined by X-ray diffractometry.
In general, mixed metal oxides are crystalline compounds and they are classified by general formulas and certain structural-type characteristics of naturally occurring minerals. Perovskite is a well-known type of mixed metal oxides. Perovskites have the general formula ABO3 where A and B stand for cations. More than one cation for each A and B may be present.
Another type of metal oxide includes xe2x80x9cperovskite-likexe2x80x9d materials which comprises basic perovskite cell separated by intervening oxide layers. Perovskite-like materials have the general formula [(ABO3)n+CyOz] where A, B and C stand for cations. More than one cation for each A, B and C may be present.
Are also known compounds derived from perovskite or perovskite-like materials by substitution and deviations to stoichiometry but maintaining their perovskite or perovskite-like crystal structure. Non-stoichiometric compounds derived from perovskites have the general formula (ABO3xe2x88x92x) and non-stoichiometric compounds derived from perovskite-like materials have the general formula [(ABO3xe2x88x92x)n+CyOz]. In all these non-stoichiometric compounds, metal ions with a different valence may replace both A and B ions thereby generating non-integral numbers of oxygen atoms in the formula. La0.8Sr0.2 CoO3xe2x88x92x and La0.8Sr0.2MnO3xe2x88x92x are examples of non-stoichiometric compounds derived from perovskites and Sr2FeO4xe2x88x92x and Sr3Fe2O7xe2x88x92x are examples of non-stoichiometric compounds derived from perovskite-like materials. Other examples of such deviation to stoichiometry are obtained by making a perovskite or a perovskite-like material deficient in oxygen. For example, the brownmillerite structure (ABO2.5) is formed from perovskites (ABO3).
It is at once apparent that there is quite a large number of compounds which fall within the scope of the term perovskite and perovskite-like materials. The compounds and their structure can be identified by X-ray diffraction.
In prior art, perovskite and perovskite-like compounds have been commonly used in the following fields: electrocatalysis, hydrogenation, dehydrogenation and auto-exhaust purification. One drawback with the metal oxides having the perovskite and perovskite-like structure produced in prior art is that, in general, they show a very low BET specific surface area (SS) in the order of 1 m2/g. Therefore despite the fact that perovskite and perovskite-like structure metal oxides are not expensive to produce, that they usually show good catalytic oxidation activities, that they are thermally stable and that they show a good resistance to poisoning, they have found to date very limited application in place of noble metal based catalysts used in the field of industrial pollution abatement or automobile emission control. Higher specific surface area perovskite and perovskite-like compounds could thus have a great potential as catalysts, particularly in the selective reduction of nitrogen oxides (NOx) and as electrocatalysts in the cathodic reduction of oxygen.
The known methods for preparing perovskites and perovskite-like materials include sol-gel process, co-precipitation, citrate complexation, pyrolysis, spray-drying and freeze-drying. In these, precursors are prepared by a humid way such as in a mixed gel or in the co-precipitation of metallic ions under the form of hydroxides, cyanides, oxalates, carbonates or citrates. These precursors can thus be submitted to various treatments such as evaporation or combustion (SSxcx9c1-4 m2/g), to the method of explosion (SS less than 30 m2/g), plasma spray-drying (SSxcx9c10-20 m2/g) and freeze-drying (SSxcx9c10-20 m2/g). However, the drawbacks with all of these methods are that either low specific surface area values are reached or that they are complicated and expensive to put into practice.
The most common method for preparing perovskite and perovskite-like catalysts is the traditional method called xe2x80x9cceramicxe2x80x9d. This method simply consists in mixing constituent powders (oxides, hydroxides or carbonates) and sintering the powder mixture thus formed to high temperature. The problem with this method is that calcination at high temperature (generally above 1000xc2x0 C.) is necessary to obtain the crystalline perovskite or perovskite-like crystalline structure. Another drawback lo is that low specific surface area value is obtained (SS around 1 m2/g). An example of such a high temperature heating method is disclosed in U.S. Pat. No. 5,093,301 where a perovskite structure to be used in a catalyst is formed after heating a ground dry powder mixture at 1300xc2x0 C.
U.S. Pat. No. 4,134,852 (Volin et al.) issued in 1979 disclosed a variant to the ceramic method by xe2x80x9cmechanically alloyingxe2x80x9d, in the old sense of that expression, the constituent powders necessary for the preparation of perovskite catalysts. Indeed, it refers to a conventional grinding in order to obtain a more or less homogenous mixture of particles but not infer any chemical reaction between the components. It can be read in column 7, lines 5-8 of this patent that xe2x80x9c[a] mechanically alloyed powder is one in which precursor components have been intimately intradispersed throughout each particle . . . xe2x80x9d. Therefore a necessary step of the process disclosed therein to obtain the desired perovskite structure is by heating the xe2x80x9cmechanically alloyedxe2x80x9d powder composition to an elevated temperature greater than 800xc2x0 C. (column 7, lines 61-62).
Today, the use of the expression xe2x80x9cmechanical alloyingxe2x80x9d or xe2x80x9cmechanosynthesisxe2x80x9d refers among other things to a high energy milling process wherein nanostructural particles of the compounds milled are induced. Therefore it also refers to the production of metastable phases, for example high temperature, high pressure or amorphous phases, from crystalline phases stable under ambient temperature and pressure. For example, the structural transformation of alumina (Al2O3), the preparation of ceramic oxides and the preparation of stabilized zirconias by high energy milling or mechanical alloying have already been respectively disclosed in the following references: P. A. Zielinski et al. in J. Mater. Res., 1993, Vol. 8. p 2985-2992; D. Michel et al., La revue de mxc3xa9tallurgie-CIT/Sciences et Gxc3xa9nies des matxc3xa9riaux, February 1993; and D. Michel et al., J. Am. Ceram. Soc., 1993, Vol 76, p 2884-2888. The publication by E. Gaffet et al. in Mat. Trans., JIM, 1995, Vol 36, (1995) p 198-209) gives an overview of the subject.
However, even if these papers disclosed the use of high energy milling, their authors have only been able to transform their starting product from one phase to another phase. The product resulting from the milling thus still has the same structure. Furthermore, none of them discloses the preparation of perovskite or perovskite-like materials.
There is still presently a need for a simple process, low in cost for producing a metal oxide having the perovskite or the perovskite-like crystal structure. Furthermore, the perovskite and perovskite-like metal oxides produced according to all of the above mentioned methods known in the art does not have a nanocrystalline structure. Therefore, there is also a need for a metal oxide having a perovskite or a perovskite-like nanocrystalline structure with a high specific surface area and need for a process for synthesizing such compounds.
An object of the present invention is to propose a process for producing a metal oxide that will satisfy the above-mentioned needs.
According to the present invention, that object is achieved with a process for mechanosynthesizing a metal oxide having a perovskite or perovskite-like crystal structure and a predetermined stoichiometric content of oxygen, said metal oxide being selected from the group consisting of perovskites of the general formula ABO3; perovskite-like materials of the general formula [(ABO3)n+CyOz]; non-stoichiometric compounds derived from perovskites and having the general formula (ABO3xe2x88x92x); and non-stoichiometric compounds derived from perovskite-like materials and having the general formula [(ABO3xe2x88x92x)n+CyOz], wherein:
A comprises at least one element selected from the group consisting of Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table;
B comprises at least one element selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo and W;
C represents at least one element selected from the group consisting of Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo, W, Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table;
n represents an integer number between 1 and 10;
0 less than x less than 3
y represents an integer number between 1 and 5;
z represents an integer number between 1 and 5;
the process comprising the step of subjecting a mixture of starting powders formulated to contain the components represented by A, B and C in the formulas to a high energy milling sufficient to induce chemical reaction of the components and thereby directly mechanosynthesize said metal oxide in the form of a perovskite or a perovskite-like material having a nanocrystalline structure as determined by X-ray diffractometry.
According to a preferred variant of the invention, the high energy milling is performed under a controlled atmosphere to control the nanocrystalline structure and the stoichiometric oxygen content of the mechanosynthesized metal oxide. The controlled atmosphere preferably comprises a gas selected from the group consisting of He, Ar, N2, O2, H2, CO, CO2, NO2, NH3, H2S and mixtures thereof.
In another preferred variant of the invention, the process is characterized in that it further comprises the step of selecting and milling the starting powders in relative portions to control the nanocrystalline structure of the mechanosynthesized metal oxide.
The present invention also provides a process for mechanosynthesizing a metal oxide having a perovskite or perovskite-like crystal structure, a predetermined stoichiometric content of oxygen, and a high BET specific surface area, said metal oxide being selected from the group consisting of perovskites of the general formula ABO3; perovskite-like materials of the general formula [(ABO3)n+CyOz]; non-stoichiometric compounds derived from perovskites and having the general formula (ABO3xe2x88x92x); and non-stoichiometric compounds derived from perovskite-like materials and having the general formula [(ABO3xe2x88x92x)n+CyOz], wherein:
A comprises at least one element selected from the group consisting of Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table;
B comprises at least one element selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo and W;
C represents at least one element selected from the group consisting of Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo, W, Al, Y, Na, K, Cs, Pb, La, Sr, Ba, Cr, Rb, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table;
n represents an integer number between 1 and 10;
0 less than x less than 3
y represents an integer number between 1 and 5;
z represents an integer number between 1 and 5;
the process comprising the steps of:
a) subjecting a mixture of starting powders formulated to contain the components represented by A, B and C in the formulas to a high energy milling sufficient to induce chemical reaction of the components and thereby directly mechanosynthesize said metal oxide in the form of a perovskite or a perovskite-like material having a nanocrystalline structure as determined by X-ray diffractometry;
b) increasing the BET specific surface area of the metal oxide obtained in step a) by further subjecting said metal oxide to high energy milling to obtain a metal oxide having a high BET specific surface area.
Step a) is preferably performed under a controlled atmosphere to control the nanocrystalline structure and the stoichiometric oxygen content of the mechanosynthesized metal oxide. Step b) is preferably performed under a controlled atmosphere to control the BET specific surface area of the mechanosynthesized metal oxide. The controlled atmospheres preferably comprise a gas selected from the group consisting of H2O, He, Ar, N2, O2, H2, CO, CO2, NO2, NH3, H2S and mixtures thereof.
The process for mechanosynthesizing a metal oxide having a perovskite or perovskite-like crystal structure, a predetermined stoichiometric content of oxygen, and a high BET specific surface area according to the invention may further comprises one or more additional steps. In another preferred embodiment, the process further comprises the step of adding a small amount of an aqueous solution to the metal oxide during the milling of step b) in order to obtain a humidified metal oxide. In another preferred embodiment, the process further comprises the step of selecting and milling the starting powders in relative portions to control the final nanocrystalline structure of the mechanosynthesized metal oxide. In an additional preferred embodiment, the process further comprises the steps of c): adding a non-reacting soluble additive during the milling of step b); and d): subsequently washing out said soluble additive. Preferably, the non-reacting soluble additive is selected from the group consisting of LiCl, NaCl, RbCl, CsCl, NH4Cl, ZnO, and NaNO3.
It is also an object of the invention to provide a metal oxide having a perovskite or a perovskite-like nanocrystalline structure and having a BET specific surface area between 3.1 and 82.5 m2/g, this metal oxide being obtained using any one of the above mentioned processes. Preferably, the metal oxide is characterized in that it consists of a brownmillerite having the formula ABO2.5 or [(ABO2.5)n+CyOz] and more particularly a brownmillerite selected from the group consisting of Sr7Fe10O22, SrFeO2.5 and SrFe0.5Co0.5O2.5.
As can be appreciated, the processes according to the present invention are simple, efficient, not expensive and do not require any heating step for producing a metal oxide having a perovskite or a perovskite-like nanocrystalline structure that may easily show a very high specific surface area. Another advantage is that the perovskite or perovskite-like obtained according to the present invention also have a nanocrystalline structure and a high density of lattice defects thereby showing a higher catalytic activity, a characteristic which is highly desirable in their eventual application as catalysts and electronic conductors. The fact that it is possible to synthesize brownmillerites using the processes of the invention is also a major advantage of the present invention.
A non-restrictive description of preferred embodiments of the present invention will now be given with reference to the appended drawings and tables.